Weight reduction of automobile bodies has become in recent years a very important issue in relation to emission control for the purpose of preserving global environments. More recently, efforts are made to achieve a higher strength of automotive steel sheets and reduce steel sheet thickness.
Because many of the body parts of automobile made of steel sheets are formed by press-working, steel sheets used are required to have an excellent press-formability. In order to achieve an excellent press-formability, it is necessary to ensure a low yield strength and a high elongation. Stretch-flanging may be frequently applied in some cases, so that it is also necessary to have a high hole-expanding ratio. In general, however, a higher strength of steel sheet leads to an increase in yield strength and deterioration of shape freezability, and tends to result in a lower elongation and a poorer hole-expanding ratio, thus leading to a lower press-formability. As a result, there as conventionally been an increasing demand for high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets and high-strength plated steel sheets having high elongation and excellent in press-formability.
Importance is now placed on safety of automobile body to protect a driver and passengers upon collision, and for this purpose, steel sheets are demanded to have an improved impact resistance as a standard of safety upon collision. For the purpose of improving impact resistance, a higher strength in a completed automobile is more favorable. There has therefore been the strongest demand for high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets and high-strength plated steel sheets having a low strength and a high elongation and excellent in press-formability upon forming automobile parts, and having a high strength and excellent in impact resistance in completed products.
To satisfy such a demand, a steel sheet high both in press-formability and strength was developed. This is a baking hardening type steel sheet of which yield stress increases by applying a baking treatment usually including holding at a high temperature of 100 to 200° C. after press forming. This steel sheet is based on a process comprising the steps of controlling the content of C remaining finally in a solid-solution state (solute C content) within an appropriate range, keeping mildness, satisfactory shape freezability and elongation during press forming, preventing movement of dislocation introduced during press forming by the residual solute C fixed to it during the baking treatment after press forming, thereby causing an increase in yield stress. However, in this baking hardening type automotive steel sheet, while yield stress can be increased, it was impossible to increase tensile strength.
Japanese Examined Patent Application Publication No. 5-24979 discloses a baking hardening high-strength cold-rolled steel sheet having a chemical composition comprising from 0.08 to 0.20% C, from 1.5 to 3.5% Mn and the balance Fe and incidental impurities, and having a structure composed of uniform bainite containing up to 5% ferrite or bainite partially containing martensite. The cold-rolled steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979 has an object to achieve a high baking hardening amount conventionally unavailable through conversion of structure from the conventional structure mainly comprising ferrite into a structure mainly comprising bainite, by rapidly cooling the steel sheet after continuous annealing within a temperature range of from 400 to 200° C. in the cooling step and then slowly cooling the same. In the steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979, however, while a high baking hardening amount conventionally unavailable is obtained through an increase in yield strength after baking, it is yet impossible to increase tensile strength, and there still remains a problem in that improvement of impact resistance cannot be expected.
On the other hand, several hot-rolled steel sheets are proposed with a view to increasing not only yield stress but also tensile strength by applying a heat treatment after press forming.
For example, Japanese Examined Patent Application Publication No. 8-23048 proposes a manufacturing method of a hot-rolled steel sheet, comprising the steps of reheating a steel containing from 0.02 to 0.13% C, up to 2.0% Si, from 0.6 to 2.5% Mn, up to 0.10% sol. Al, and from 0.0080 to 0.0250% N to a temperature of at least 1,100° C., applying a hot rolling end finish rolling at a temperature of from 850 to 950° C., then cooling the hot-rolled steel sheet at a cooling rate of at least 15° C./second to a temperature of under 150° C., and coiling the same, thereby achieving a composite structure mainly comprising ferrite and martensite. In the steel sheet manufactured by the technique disclosed in Japanese Examined Patent Application Publication No. 8-23048, however, while tensile strength is increased, together with yield stress, by strain age hardening, a serious problem is posed in that coiling of the steel sheet at a very low coiling temperature as under 150° C. results in large dispersions of mechanical properties. Another problems include large dispersions of increment of yield stress after press forming and baking treatments, as well as an insufficient press-formability resulting from a low hole-expanding ratio (λ) and a decreased stretch-flanging workability.
On the other hand, for some portions, automotive parts are required to have a high corrosion resistance. A hot-dip galvanized steel sheet is suitable as a material applied to portions required to have a high corrosion resistance, and a particular demand exists for hot-dip galvanized steel sheets excellent in press-formability during forming, and is considerably hardened by a heat treatment after forming.
To respond to such a demand, for example Japanese Patent Publication No. 2802513 proposes a manufacturing method of a hot-dip galvanized steel sheet using a hot-rolled steel sheet as a substrate. The patented method comprises the steps of hot-rolling a steel slab containing up to 0.05% C, from 0.05 to 0.5% Mn, up to 0.1% Al and from 0.8 to 2.0% Cu under conditions including a coiling temperature of up to 530° C., reducing the steel sheet surface by heating the hot-rolled steel sheet to a temperature of up to 530° C., and hot-dip-galvanizing the sheet, whereby a remarkable hardening is available through a heat treatment after forming. In the steel sheet manufactured by this method, however, in order to obtain a remarkable hardening from the heat treatment after forming, the heat treatment temperature must be at least 500° C., and this has posed a problem in practice.
Japanese Unexamined Patent Application Publication No. 10-310824 proposes a manufacturing method of an alloyed hot-dip galvanized steel sheet permitting expectation of an increase in strength through a heat treatment after forming, using a hot-rolled or cold-rolled steel sheet as a substrate. This method comprises the steps of hot-rolling a steel containing from 0.01 to 0.08% C, appropriate amounts of Si, Mn, P, S, Al and N, and one or more of Cr, W and Mo in a total amount of from 0.05 to 3.0%, or cold-rolling or temper-rolling the sheet and annealing the same, applying hot-dip galvanizing the sheet, and then, conducting a heating/alloyinq treatment. The Publication asserts that, after forming, tensile strength is increased by heating the sheet at a temperature within a range of from 200 to 450° C. However, the resultant steel sheet involves a problem in that, because the microstructure comprises a ferrite single phase, a ferrite+pearlite, or a ferrite+bainite structure, a high elongation and a low yield strength are unavailable, resulting in a low press-formability.
Japanese Unexamined Patent Application Publication No. 11-199975 proposes a hot-rolled steel sheet for working excellent in fatigue property, containing from 0.03 to 2.0% C, appropriate amounts of Si, Mn, P, S and Al, from 0.2 to 2.0% Cu, and from 0.0002 to 0.002% B, of which the microstructure is a composite structure having ferrite as a main phase and martensite as the second phase, and the state of presence of Cu in the ferrite phase in a solid-solution state and/or precipitation of up to 2 nm. The proposed steel sheet has an object based on a fact that fatigue limit ratio is remarkably improved only when compositely adding Cu and B, and achieving the finest state of Cu as up to 2 nm. For this purpose, it is essential to end hot finish rolling at a temperature of at least the Ar3 transformation point, air-cool the sheet within a temperature region of from Ar3 to Ar1 in cooling for a period of from 1 to 10 seconds, then cool the sheet at a cooling rate of at least 20° C./second, and coil the cooled sheet at a temperature of up to 350° C. A low coiling temperature of up to 350° C. poses a problem of causing a serious deformation of the shape of the hot-rolled steel sheet, thus preventing industrially stable manufacture.